po: Assume a 'sed' that supports '#' for introducing comments.

po: Assume a 'sed' that supports '#' for introducing comments.

This is standardized in POSIX since POSIX:2001 (= SUSv3).
It is supported on all platforms that I could test:
GNU, macOS 10.5, FreeBSD 5, NetBSD 7, OpenBSD 6, AIX 7, Solaris 10 and 11,
busybox (2019).

* gettext-runtime/po/remove-potcdate.sed: Renamed from
gettext-runtime/po/remove-potcdate.sin.
* gettext-runtime/po/Makefile.in.in (DISTFILES.common): Update.
(.SUFFIXES): Remove .sin.
(.sin.sed): Remove rule.
($(DOMAIN).pot-update): Use remove-potcdate.sed in $(srcdir).
(mostlyclean): Don't remove remove-potcdate.sed.
* autogen.sh: Update.
* Makefile.am (distcheck-hook): Update.
* gettext-tools/examples/po/remove-potcdate.sed: Renamed from
gettext-tools/examples/po/remove-potcdate.sin.
* gettext-tools/examples/po/Makefile.am ($(DOMAIN).pot-update): Use
remove-potcdate.sed in $(srcdir).
(MOSTLYCLEANFILES): Don't add remove-potcdate.sed.
(EXTRA_DIST): Update.
* gettext-tools/examples/hello-*/po/Makefile.am ($(DOMAIN).pot-update): Use
remove-potcdate.sed in $(srcdir).
(MOSTLYCLEANFILES): Don't add remove-potcdate.sed.
(EXTRA_DIST): Update.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile
($(DOMAIN).pot-update): Remove dependency on remove-potcdate.sed.
(internal-clean): Remove rule.
* gettext-tools/examples/hello-*/autogen.sh: Update.
* gettext-tools/examples/hello-*/autoclean.sh: Update.